Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim f As New Form
        Me.AddOwnedForm(f)
        f.Show()
    End Sub
 
    Private Sub Form1_Closing(ByVal sender As Object, ByVal e As _
      System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
        Dim res As Boolean = (Me.OwnedForms.Length > 0)
        Debug.WriteLine(res)
        e.Cancel = res
    End Sub